As is appreciated in the arts of psychology and cognitive science, emotion is a subjective, conscious experience that is primarily characterized by psycho-physiological expressions, biological reactions, and mental states. The physiology of emotion is closely linked to the arousal of the nervous system, with various states and strengths of arousal corresponding to particular emotions. In other words, emotion is a complex state of feeling that results in physical and psychological changes that can influence a person's behavior and the behavior of others that the person interacts with. Emotion is also linked to behavioral tendency. For example, extroverted people are more likely to outwardly express their emotions, while introverted people are more likely to conceal their emotions. Over the past two decades research on emotion has increased significantly in a number of different fields such as psychology, neuroscience, endocrinology, medicine, history, and sociology. There is a well-known correlation between a person's emotional state and their mental well-being. There is also a well-known correlation between a person's emotional state and their physical health.